1. Field
Various embodiments of the present invention relate to an image sensor, and more particularly, to a pixel signal read-out technology in a CMOS image sensor.
2. Description of the Related Art
Complementary metal oxide semiconductor (CMOS) image sensors need to have high speed and high density and therefore difficult to secure sufficient time for reading out pixel data.
Specifically, a transmission of image data (i.e., pixel data), outputted from a pixel array, to an analog-to-digital converter (ADC) with correlated double sampling (CDS) scheme takes long time.
FIG. 1 is a diagram illustrating a conventional pixel of a CMOS image sensor. FIG. 1 illustrates the structure of a 4-transistor pixel of the CMOS image sensor, which is a well-known art.
As illustrated in FIG. 1, in the conventional pixel structure, a voltage accumulated in a photodiode (PD) is transferred to a floating diffusion node FD, which is an input node (i.e., a gate terminal) of a pixel source follower M2 and M3, through a transfer transistor M1 in response to a transfer signal TX. The voltage transferred to the floating diffusion node FD changes an output voltage of the pixel source follower M2 and M3, which corresponds to the output of a pixel. For reference, a reset transistor M4 resets the voltage of the floating diffusion node FD in response to a reset signal RX, and a selection transistor M3 outputs the output voltage to a pixel output node VPXO, in response to a selection signal SX.
However, since many pixels included in a pixel array are coupled to one pixel output node, loading capacitance is relatively great, and as the pixel density of a CMOS image sensor increases, the loading capacitance also increases.
Due to such loading capacitance, much pixel conversion time is required to transfer the voltage of the floating diffusion node FD to an analog-to-digital converter.
As a frame per second (FPS), i.e., the number of image frames per second of a CMOS image sensor and pixel density increase, the pixel conversion time becomes insufficient.
When the pixel conversion time is insufficient, a pixel signal may not be transferred correctly to the analog-to-digital converter, and thus a noise may occur in the CMOS image sensor.